Plenum spaces (drop ceilings or spaces below the floor) commonly contain lighting fixtures, heating and air-conditioning ducts, wiring for electrical communication, along with alarm and security circuits.  Because these areas can be high in oxygen content during a fire, current Electrical and Building Codes mandate that wires installed in plenum spaces must be inside a metal conduit or Plenum Rated cables.  Plenum Rated or FT6 cables are designed to produce little smoke and are fire resistant, making them, therefore, compliant with the required codes.